Lithium ion batteries are used in many consumer applications due to their high energy density and their long lifetime. Lithium thin film batteries differ from Lithium ion batteries in several important ways. Thin film batteries use a solid electrolyte instead of the liquid or gel electrolyte used in Lithium ion and Lithium polymer batteries (U.S. Pat. No. 5,338,625). The solid electrolyte prevents dendimer growth from a Lithium anode, so unlike Lithium ion or Lithium polymer batteries, a Lithium anode can be used in thin film batteries in rechargeable applications. The use of a Lithium anode can increase the theoretical capacity of the battery ten times compared to a Lithium ion cell. The solid electrolyte also makes packaging the cell much easier. With a liquid electrolyte, a ‘can’ is used to encapsulate the battery and seal in the electrolyte. As the size of the battery is scaled down, a larger fraction of the volume is needed for this packaging, thereby reducing the energy density of the battery. With the solid electrolyte of a Lithium thin film battery, a very thin encapsulation layer is used to prevent oxidation of the Lithium anode. Thus, thin film batteries can be scaled down more efficiently than Lithium ion or Lithium polymer batteries. Furthermore, the solid electrolyte provides a very stable interface between the anode and the cathode. Due to this stability, the cycleability of the cell is significantly improved by between 10× to 100×, compared to Lithium ion batteries.
One of the key drawbacks to Lithium thin film batteries is their scalability. They scale very well to small dimensions, but for applications that demand higher areal energy density, their scaling is limited. As Lithium ions are moved from the anode to the cathode and intercalated into the cathode, the volume of the cathode changes. Since the electrolyte is a solid, it is difficult to accommodate these volume changes and ionic conductivity is reduced. As the cathode becomes thicker, the stresses due to the volume expansion become worse. This mechanism will eventually limit the capacity that can be gained by scaling up the thickness of the cathode. When the cathode thickness exceeds about 10 micrometers, the power performance of the cell drops dramatically.
One known method of increasing the capacity of thin film batteries per unit area, taught in U.S. Pat. No. 5,612,152, comprises stacking multiple cells on top of each other. The cells are stacked by depositing the layers sequentially with almost a ten fold increase in capacity, however, stress buildup and cost limit further stacking. Stacking can also be accomplished with a packaged cell, but cost and capacity density are issues with this approach. A further method involves forming the battery on the sidewalls of high aspect ratio trenches (PCT Publication Number WO 2005/027245 A2). This approach offers good scalability for small increases in cost; however, the solid electrolyte is deposited by physical vapor deposition in which conformal coating can not be achieved on sidewalls, preventing fabrication of batteries.
Accordingly, it is desirable to provide an monolithically integrated Lithium thin film battery that provides increased areal capacity on a single level (without stacking of multiple cells). Furthermore, other desirable features and characteristics of the present invention will become apparent from the subsequent detailed description of the invention and the appended claims, taken in conjunction with the accompanying drawings and this background of the invention.